Transparent typewriter bails have been described in the art. Such bails have a transparent bail bar through which printed lines can be read. Paper contacting rollers which can be slidably moved along the bar contact underlying paper to be typed on.
An early disclosure of a transparent bail can be found in the German Pat. No. 719,558. As described therein, a transparent bail is provided capable of magnifying the characters typed beneath the bail to facilitate operator reading of the printed line. Transparent bails for a typewriter are described in the Swiss Pat. Nos. 244,359 and 278,614.
In my prior U.S. Pat. No. 2,772,765, dated Dec. 4, 1956, a transparent bail of generally rectangular cross-sections is described to enable direct viewing of a printed line beneath the bail. The transparent bar used with the bail is provided with side located reinforcing strips. This bail, although effective, is difficult and expensive to manufacture, assemble and install.
Conventional typewriter bails have a metal cross bar on which paper rollers can slide. Spring loaded support structures are used at the bar ends to bias the bail against the platen. The spring bias frequently is not evenly applied so that one end of the metal bar may be subjected to a greater clamping force than the other end. Metal cross bars, however, may tolerate such uneqal end loads while retaining proper operational contact between the paper rollers and the paper overlying the platen.
The flexible nature of a rectangular transparent plastic bar used in transparent bails permits undesirable bending or bowing of the bail causing lifting of the center segment. As a result, contact between the paper rollers and paper beneath the bail tends to depend upon the roller position along the bar. When contact between one paper roller and the paper is present but not with the other roller, uneven paper printing occurs, particularly near the bottom of the page where the paper rollers are relied upon to maintain alignment of the paper.
With a transparent bar formed of plastic, a bowed mounting in a typewriter tends to result in a permanent set over a period of time. Unequal spring loading by the bail support structure tends to aggravate such bowing effects on plastic cross bars and eventually causes malfunctioning of the transparent typewriter bail by virtue of a cold flow of the plastic.
In the manufacture of typewriters, it is not currently the practice to employ transparent bails. Hence, it is desirable to be able to install a transparent bail in an existing typewriter in a convenient and quick manner. It is particularly desirable to be able to adjust the transparent bail so that it is suitable to the individual operator for enhanced visibility of the print line underneath the bail.